Over a million miles of small diameter, e.g. less than 1.25″ nominal (1.66″ OD), underground transmission pipe exists in North America at this time. All of this pipe is aging, and in many cases the aging process results in increased transmission pressure losses along with degradation of the ductile properties of the pipe, necessitating replacement of the pipe. Methods for replacing easily slit plastic and copper piping are disclosed in Crane et al. U.S. Pat. No. 6,171,026, issued Jan. 9, 2001 for a “Method and Apparatus for Replacement of Pipelines, and Crane et al. U.S. Pat. No. 6,655,874, issued Dec. 2, 2003 for a “Method and Apparatus for Slitting Flexible Pipe in an Underground Bore.”
Unfortunately, not all of the million plus miles of aging small diameter transmission pipe is easily slit copper or plastic. Many of the underground lines are lead or steel pipe. Although lead and steel can be slit, the use of conventional slitting techniques is limited in the case of small diameter pipes because the force required to slit the pipe is high and the column strength of the pipe, especially lead pipe, is low. As used herein, “small diameter” refers to pipes having an outside diameter of less than 1.66 inches, while the term “column strength” refers to the resistance of a structure to collapse when compressive force is applied. The method described in the above-referenced U.S. Pat. No. 6,655,874 deals with the lack of column strength of underground pipe during the slitting operation by allowing induced tensile stress to be a balancing force that holds the pipe in place while the pipe is slit from the inside.
In some cases it is desirable to extract the old pipe during the replacement operation. The method disclosed in the above-referenced U.S. Pat. No. 6,171,026 demonstrates removal of the pipe with simultaneous installation of a new product pipe. However, a need remains for a method effective for slitting and replacement of lead or steel underground pipes.